1. Field of the Invention
This invention relates to an image transfer method and apparatus, and more specifically to improving the quality of an image transferred to an image receiving medium.
2. Description of the Prior Art
The use of a fiber optic cathode ray tube (CRT) in a hard copy recording system is well known. The CRT provides a luminous signal which is received by a light sensitive medium and a permanent visual representation of the signal is formed. The signal represents image information stored on a line by line basis. The luminous signal is created when a beam of electrons strikes a phosphorescent stripe on the inner side of the CRT face plate. As the electron beam scans the phosphor stripe from one end of the faceplate to the other, the phosphor coating will emit light which will in turn impinge on the image receiving medium as it moves past the CRT face plate. The scrolling of a image receiving medium is synchronized with the transfer of the image information from the CPU, and its movement is substantially perpendicular to the scanning plane of the electron beam.
In most of the prior art, only one phosphor stripe is used on the faceplate of the CRT. The electron beam repeatedly traces a single track along the phosphor stripe which exposes the image receiving medium. This technique can lead to image quality problems. One problem comes about because of variations in the thickness of the phosphor stripes. During manufacture of the CRT, the thickness of the phosphor applied to the CRT screen may vary causing the light emitted during the operation to be of greater intensity in the places where the phosphor is the thickest. This may lead to undesirable artifacts appearing in the exposed image. These artifacts will appear on every line of the exposed image if the one track technique of the prior art is used. Another problem with the single track CRT recorders is that as the track is continuously activated, the phosphor along the track will in time lose its efficiency and a line will be permanently burned into the phosphor,
One solution was disclosed in U.S. Pat. No. 4,499,501 issued on Feb. 12, 1985 to Joern B. Eriksen which uses multiple tracks. As there described, each line on the image receiving medium is exposed by each track on the phosphor stripe in the CRT. As the image receiving medium is scrolled past the faceplate, each line on the image receiving medium will be exposed the same number of times that there are tracks, and each exposure is only a fraction of the full exposure needed. This prior art proposes a solution to the problem of repeatedly exposing any deficiency in a transmission path, but since every transmission path is continuously activated the problem with phosphor ageing still exists.